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nished also 27. On treatment with TsOH/MeOH, 27 was converted to luotonin E 28. Similar synthesis of luotonins A, B, and E without use of lithiation has been reported also by Chavan and Sivappa [282]. They succeeded acid-catalyzed cyclodehydration of 121 to afford 26 without use of the Mitsunobu conditions that require laborious column chromatography to obtain pure luotonin A.

The route III involves an efficient total synthesis of luotonin A via intramolecular hetero Diels-Alder reactions by Ihara et al. [283,284], and several syntheses of luotonin A utilizing well-known Kametani's quina-zolinone synthesis via iminoketene cycloadditions or analogues as the final key-step by several groups [229,285-288] (Scheme 29). In the hetero Diels—Alder route, a key precursor, N-(2-cyanoquinolin-3-ylmethyl)-2-methoxybenzamide (122) prepared from the corresponding 2-bromo derivative by palladium-catalyzed coupling with copper (I) cyanide underwent the intramolecular hetero Diels-Alder reaction via a putative intermediate 123 in the presence of chlorotrimethylsilane and zinc chloride to furnish luotonin A (Scheme 29). On the other hand, first total synthesis of luotonin A via